1. Field of the Invention
The invention relates to single-frequency lasers, specifically, to monolithic laser diodes including at least one distributed Bragg reflector (DBR), and more specifically, to distributed feedback (DFB) laser diodes. The invention also relates to the selective oxidation of Al-rich III-V layers, specifically for laterally oxidized ridge lasers.
2. Description of Related Art
Distributed feedback (DFB) laser diodes are monolithic, compact, power-efficient and low-cost sources of coherent light. The lasers are capable of emitting a single narrow low-noise spectral line, which can be smoothly tuned by either carrier injection or temperature without hopping. Moreover, if properly constructed, the devices can be directly modulated with the frequencies up to few tens GHz and remain in single-line operation. Owing to these features, DFB lasers are applied in various areas including, but not limited to, communications, solid-state laser pumping, and spectroscopy.
Like a simple Fabri-Perot laser diode, the construction of a DFB laser diode includes (see, for example, G. P. Agrawal and N. K. Dutta, “Semiconductor Lasers”, Van Nostrand Reinhold, 1993, pp. 181-212, 321-323) a semiconductor heterostructure, which is designed for confinement of both photons and charge carriers in the vertical direction. In order to confine photons, the heterostructure includes an optical waveguide ordinarily supporting only one fundamental mode. The waveguide includes a core surrounded by claddings having smaller refractive indexes compared to the core index. The core includes an active region usually consisting of one or a few quantum-wells and providing a separate carrier confinement. Being pumped over the transparency level, the active region operates as a gain medium for the guided light. The cladding layers are doped oppositely, by donors and acceptors, while the core normally remains undoped, so that a p-i-n junction is formed. When the p-i-n junction is biased in the forward direction, electrons and holes are injected into the active region, thereby providing an inverse population of energy states. Ohmic metallic contacts are formed from the top and bottom sides of the heterostructure and allow the application of current, as well as mounting the laser diode on a holder and wiring.
DFB lasers can be broad area, e.g. for pumping, or include also a lateral confinement, e.g. for communications. A vast majority of commercial DFB lasers with lateral confinement are index-guided and use buried heterostructures for strong index-guiding or ridge waveguides for weak index-guiding. Typically, buried heterostructure lasers have better performance in terms of the threshold current, the slope efficiency and the modulation rate, mainly because they have a narrow stripe and are not affected by lateral carrier diffusion. Therefore, these lasers are used in communications. However, the buried heterostructure type waveguides are complicated and, therefore, more critical to the fabrication process. They require at least one overgrowth step that results in higher cost of manufacturing and lower yield. Ridge waveguide designs are simpler and more tolerant of the fabrication process. They allow control of a lateral index step on a post-growth stage of laser processing and can be realized without the overgrowth.
The distinguishing feature of the DFB lasers as compared to other laser diodes is a presence of the integrated periodical Bragg grating structure. Being placed close to or inside the waveguide core, the grating couples counter-propagating optical waves having the wavelength within the grating stop-band, i.e. in a certain vicinity of the Bragg wavelength λB,m=2 neΛ/m, where ne is the mode effective refractive index, Λ is the grating period, and m is a natural number defining the diffraction order for back reflected radiation. Such a grating works as a spectrally selective distributed mirror and is capable of providing an optical feedback for laser generation. In order to reach the generation state, the grating period is selected such that the optical gain at the Bragg wavelength is positive at a certain pumping level and covers summary optical loss.
The Bragg grating of a conventional DFB laser is embedded into the optical waveguide cladding either above the core (see, for example, T. R. Chen et al., “Very high power InGaAsP/InP distributed feedback lasers at 1550 nm wavelength,” Appl. Phys. Lett., vol. 72, pp. 1269-1271, 1998; K. Nakahara et al., “40-Gb/s direct modulation with high extinction ratio operation of 1.3-μm InGaAlAs multiquantum well ridge waveguide distributed feedback lasers,” IEEE Photon. Technol. Lett., vol. 19, pp. 1436-1438, 2007), or below the core (M. Oishi et al., “MOVPE-grown 1.5 μm distributed feedback lasers on corrugated InP substrates”. IEEE J. Quantum. Electron., QE-23, pp. 822-827, 1987; K. Otsubo et al., “1.3-μm AlGaInAs Multiple-Quantum-Well Semi-insulating Buried-Heterostructure Distributed-Feedback Lasers for High-Speed Direct Modulation,” IEEE J. Sel. Top. Quantum Electron., vol. 15, pp. 687-693, 2009) that provides vertical or, in other words, transverse coupling geometry. Advantages of the transverse coupling geometry are the large coupling strength possibility, the precise control of the overlap between the grating and the optical mode, and only a slight dependence of the coupling coefficient on the lateral waveguide shape and width. The possibility of large coupling strength is applicable to low-threshold DFB lasers having short cavities and also favors a higher side mode suppression ratio. The precise control of the overlap between the grating and the optical mode relates to a better reproducibility of laser characteristics. The slight dependence of the coupling coefficient on the lateral waveguide shape and width allows using either narrow or non-narrow waveguides, which is critical for high-power DFB lasers. However, a serious disadvantage of the deep grating incorporation into the heterostructure is a strong necessity of at least one regrowth step over the grating formed on the surface of the epitaxial heterostructure. The regrowth step considerably complicates the device fabrication process and ultimately increases costs. The qualitative regrowth is especially difficult in Al-containing material systems, e.g. AlGaAs/GaAs and AlGaAsSb/GaSb.
Laterally coupled DFB (LC-DFB) lasers, first demonstrated by L. M. Miller (L. M. Miller et al., “A distributed feedback ridge waveguide quantum well heterostructure laser,” IEEE Photon. Technol. Lett., vol. 3, pp. 6-8, 1991), avoid problems induced by the overgrowth. The grating of the LC-DFB lasers is created on a structure surface in close proximity to the ridge or patterned onto the side walls, whereas the current flows in the vertical direction. Since there is no need for carrier flow through the grating, the lasers can be manufactured with a single-step growth. The LC-DFB lasers can be either index-coupled (see, for example, L. M. Miller et al., “A distributed feedback ridge waveguide quantum well heterostructure laser,” IEEE Photon. Technol. Lett., vol. 3, pp. 6-8, 1991; R. D. Martin et al., “CW performance of an InGaAs—GaAs—AlGaAs laterally-coupled distributed feedback (LC-DFB) ridge laser diode,” IEEE Photon. Technol. Lett., vol. 7, pp. 244-246, 1995) or gain (loss)-coupled. The gain (loss)-coupled lasers exploit a periodically modulated imaginary part of the refractive index instead of the real part. One way to fabricate such a laser utilizes a metal-stripe grating instead of the surface corrugated grating (see, for example, S. J. Jang et al., “Laterally coupled DFB lasers with self-aligned metal surface grating by holographic lithography”, IEEE Photon. Technol. Lett., vol. 20, pp. 514-516, 2008). As the pure gain (loss) coupling is not so simple to reach, a more correct name is complex-coupled. The LC-DFB lasers based on complex coupling do not suffer from longitudinal mode degeneration, so that there is no need in λ/4 phase shift or modulated stripe width to guarantee stable single-longitudinal mode lasing, and these lasers are less sensitive to the optical feedback. However, since there is an inherent loss mechanism, they demonstrate less power efficiency compared to the index-coupled lasers. Both index-coupled and complex-coupled LC-DFB lasers have low overlap of the grating with the optical field that limits the upper achievable value of the coupling coefficient. Another drawback is the complicated post-growth processing procedure, which requires either deep grating etching before forming the ridge or precise lithography on a nonplanar surface after forming the ridge. The former is extremely difficult for the first-order gratings. In the case of the latter, the coupling coefficient strongly depends on the mesa depth and shape, e.g. mesa walls tilt and footing, which have to be accurately controlled.
Thus, all of the prior art DFB lasers suffer from at least one of the following drawbacks: multi-step structure growth, the complicated and precise post-growth technology need, a low grating coupling coefficient, and low power efficiency.
All of the above-mentioned references are hereby incorporated by reference herein.